This invention relates to glass sheets adapted for the production of glazing panes suitable for mounting in automobile vehicles and industrial vehicles and, more specifically, for serving as lateral window panes.
The panes used in this latter type of application must satisfy the legal requirements relating to their light transmission. Thus, panes intended for the production of lateral windows must have a global light transmission factor under illuminant A (TLA) of at least 70%.
The glazed area of automobile vehicles being at present very large and the demands of clients with regard to comfort becoming more and more exacting, the manufacturers of these vehicles are continually seeking every means that will make possible a reduction in the sensation of heat experienced by passengers subjected to the solar radiation. At the same time, however, the manufacturers of automobile vehicles are endeavouring to reduce as far as possible the weight of all the glass equipment.
For the purpose of maintaining a high light transmission in the visible part of the spectrum while absorbing the maximum possible proportion of the remainder of the solar energy, it is known to introduce iron into the composition of the glass used for the production of the sheets. The iron is present in the glass both in the form of ferric oxide (Fe2O3) and ferrous oxide (FeO). The presence of Fe2O3 makes possible the absorption of the UV radiation and that which has the shortest wavelengths in the visible part of the spectrum; on the other hand, the presence of FeO makes possible the absorption of the radiation of the near infrared and that corresponding to the long wavelengths of the visible range. If the increase of the iron content, in both forms of oxide, accentuates the absorption of the radiation at the two extremities of the visible spectrum, this effect is achieved to the detriment of the transmission of light.
At present, various solutions have been proposed for utilizing to the optimum the capacity of the iron oxides for absorbing radiation while nevertheless conserving the greatest possible transmission of light.
Thus, patent EP-B-297 404 describes and claims silico-sodo-calcic glasses in which the total iron content, expressed in the form of Fe2O3, lies between 0.45 and 0.65%. These glasses are manufactured in conditions such that at least 35%, and preferably at least 50%, of total iron is in the form of FeO. The increase in the FeO content thus obtained enables the absorption of the glasses in the infrared to be accentuated and the global energy transmission factor (TE) to be reduced. However, when a glass is manufactured in the presence of sulphur in highly reducing conditions, it will have an amber colour due to the formation of chromophores which result from the reaction between the sulphur and the ferric iron. In order to avoid this, it is therefore necessary to eliminate the sulphates from the vitrifiable mixture and, since the sulphur content in a glass is never zero, to ensure that the percentage of ferric iron remains low, which implies rigorously limiting the total iron content. It follows that the capacity of these glasses for absorbing UV radiation is mediocre.
It is also known to produce glasses which, as a result of a higher total iron content than that specified by the above-mentioned European patent, combine a good transmission of light and a good absorption of the infra-red and ultraviolet radiation.
Thus, patent U.S. Pat No. 5,214,008 describes glasses devoid of ceric oxide and other oxides of that type, which contain from 0.7 to 0.95% by weight total iron expressed in the form Fe2O3. These glasses are produced in conventional furnaces from ordinary vitrifiable raw materials. The degree of oxidoreduction of the glass is controlled by the introduction of carbon and sodium sulphate into the vitrifiable mixture.
This oxidoreduction degree varies within precise limits such that the iron in the form of FeO in the glass varies from 0.19 to 0.24% by weight, said glass having, for a thickness of from 3.7 to 4.8 millimetres, a light transmission factor higher than 70%, a transmission in the ultraviolet of less than 38% and a global energy transmission factor of less than 44.5%.
Other silico-sodo-calcic glass compositions make possible the obtaining, for a given thickness, of a light transmission factor of at least 70% and a good absorption of the infrared and ultraviolet radiations. This is the case, notably, of those described in patent applications EP-A-488 110 and WO-91/07356. In addition to the oxides of iron, the glasses specified in these patent applications contain ceric oxide and titanium oxide.
The present invention has as its subject a glass sheet formed from a glass capable of being spread over the surface of a bath of molten metal, the transmission characteristics of which are principally controlled by the presence of iron oxides and which have, in comparison with glass sheets possessing a comparable overall light transmission factor, a capacity for absorption of the infrared and ultraviolet radiations at least equivalent to that of said glasses but with a smaller thickness.
The present invention also has as its subject glass sheets making possible the production of lateral glazing panes for automobile vehicles, the thickness of which is smaller than the thickness of the known lateral panes, but which nevertheless have comparable transmission characteristics.
The objective of the invention is achieved by a glass sheet formed of a silico-sodo-calcic glass which contains, expressed in percentages by weight, from 0.85 to 2% total iron expressed in the form of Fe2O3, the content by weight of ferrous iron in the form FeO being from 0.21 to 0.40%, said glass having, for a thickness of from 2 to 3 mm, a global light transmission factor under illuminant A (TLA) of at least 70%, a global energy transmission factor (TE) of less than approximately 50% and a transmission factor of ultraviolet radiation less than approximately 25%. The light transmission and energy transmission values have been determined by the Parry Moon Mass 2 method; the transmission in the ultraviolet was determined by the method defined in standard ISO 9050.